1. Field of the Invention
This invention relates to molds in which glass articles are blown rotationally or non-rotationally.
2. Description of the Prior Art
In the production of hollow glass articles, a method of rotationally blowing glass articles molds has been well known in. A mold used in such a method is usually referred to as a paste mold, as disclosed in U.S. Pat. No. 3,350,189 and British Pat. No. 1,084,816, and is provided on its inner molding surface with a lining consisting of a paste layer.
The paste layer is porous and can absorb water so that a steam cushion is formed between the glass and the mold due to a high temperature of the glass parison. Thereby, the glass is prevented from directly contacting and sticking to the mold during the blowing operation so that the molded article can be readily released from the mold and has a smooth and glossy surface.
A known typical paste layer is formed by coating a paste on the inner surface of a mold of cast iron, for example sticking cork powder onto the paste and baking them to form a carbonized cork layer on the inner surface of the mold. The carbonized cork layer is porous and can absorb and retain water.
However, since the paste layer, such as a carbonized cork layer, is abraded and burned in use, its water absorbability or chargeability degrades so that the steam cushion is formed insufficiently after prolonged use to develop scars and grains on the surface of molded glass article. For this reason, the deteriorated carbonized cork layer should be removed prior to the development of such surface defects and a fresh carbonized cork layer should be applied. Such a paste mold has thus disadvantages that it is low in working ratio and high in maintenance cost.
In addition, such a paste mold has a problem that fine carbonized cork pieces may be stuck firmly on the surface of molded glass product.
In order to remove those disadvantages of the paste mold, blow molds are proposed in Japanese Patent Publication No. 10438/'62 and Japanese Utility Model Publication No. 9726/'65, wherein the mold is made of a sintered alloy and is provided with no paste layer.
Since a sintered alloy has high porosity, the sintered alloy mold having no paste layer can retain water in the pores. Accordingly, a steam cushion can be formed between the glass surface and the inner surface of the mold during the blowing operation. In addition, since the alloy is exposed to the inner surface of the mold, the mold is subjected to neither abrasion nor damage during blowing operation so that the working life is drastically extended as compared with the paste mold. The working ratio and the maintenance cost are also improved.
However, in the sintered alloy mold, those pores are formed in a network form by connection of gaps between the sintered alloy grains so that the outer and inner surfaces of the mold are connected through such pores. Hence, the steam formed during the blowing operation leaks through the pores to the exterior of the mold so that a sufficient steam cushion cannot be formed between the molded glass surface and the inner surface of the mold. In addition, since the pores are very fine, they are apt to be plugged by hardness components and other fine contaminants in water. This means reduction of the amount of leaking steam, but the water amount absorbed and retained in the pores of the mold is also reduced so that a sufficient steam cushion cannot be formed. Furthermore, once the pores are plugged, the mold cannot be reused so that the working life of the mold is shortened. Therefore, the sintered alloy mold is uneconomical in view of the fact that the sintered alloy is expensive.
Still further, since the sintered alloy is brittle, the sintered alloy mold is easily broken.
In the production of asymmetric hollow glass articles and articles having a surface pattern, a mold is usually used in the blowing process but no rotation is given to the glass parisons.